Since the existing patio is on a granular base (as opposed to solid concrete) I would absolutely, positively NOT consider any mortared work on top of it. The reason is that the earth is a flexible surface. Though it moves so slowly you can't see it, it is doing it nevertheless. If you were to mortar flagstone or thin brick on top of it, this work would effectively be only as thick as the mortar that glued it on ... 1/2" maybe (?) It wouldn't be thick enough to support itself as the earth underneath shifts. If you're going to mortar something on top, you need a 4" solid concrete, driveway-like base (3 1/2" actual) that will not flex at all. But you could go over it with regular thickness pavers on sand IF you insulate the two layers with an inch of sand. (This is predicated on knowing that the existing base is of SOLID compacted material. Since it's been there a good while, its present shape is a good indicator of how solid the base is.) I would go so far as to say that if the present patio was perfect condition -- smooth, of perfect grade, and all slab joints perfectly matched -- you could go over it without the sand insulation. But it's not likely that it's that perfect. If you go over it, one thing you'll need to pre-plan carefully is the finish grade. It looks like your grade might end up being too high. I don't know what all the holes at the foundation are for, but they're probably not wanting to be blocked. If you need a lower grade, sell all those slabs on craigslist (someone would probably love to have them ... they look like step treads, BTW) and build directly onto your already existing base.

Check Home Depot for LARGE cloth containers of sharp sand or "base material". You might need to reset a couple of pavers if you get frost heave, but unless you have hard-frozen ground deeper than the base, it should work.

Hi, I thought I would pop in and add my 2 cents here. First let me tell you that when I was a kid my dad put in a sunken patio. It was 18" below grade and he put down sand and then layed in brick. We had very heavy clay soil so it was thought that settling wouldn't be much of an issue. After a few years the bricks had settled here and there, with standing water after steady rains pooled in the low spots and took quite a while to drain off resulting in some areas accumulating moss. A little slick to walk on and a bit of a hazard after a long time. The bricks were also no longer tightly layed next to each other either. After less than 10 years he ended up laying a piece of outdoor carpet over the bricks to hide the unsightly moss and reduce the chance of slipping. Even with scrubbing the moss regularly with bleach to remove it he ended up losing the battle. My husband (a bricklayer by trade) put a new patio in at our home and used a limestone screen base. It has been 2 years and there is no settling at this point. Weeds are at a minimum. Occasionally we will have a few pop up but they have been very easy to pull. We used large irregular sized pieces of sandstone for the patio that were reclaimed old sidewalk squares that we decided to break and make into an asymetrical pattern with a brick spiral in the center. So far so good with the crushed limestone base. Excellent drainage, no standing water even after heavy rains. we did install a french drain below the crushed stone to promote even better drainage as well though.

It's not common, but it is done. I'm presuming that they'll use a dry mix and plate compact it? Much depends on the thickness they are laying and what is under the portland/sand dry mix. There needs to be good gravel drainage and NO organic matter (no topsoil, roots, etc). The dry mix will cure, as over time it will absorb moisture from the ground and surrounding environment. My preference is to not use a portland-based base unless the pavers will be mortared in place. If the pavers will be dry-set, then I prefer a base of stone dust, which is crushed granite. Stone dust provides an easily compacted base that can still drain. The draining aspect of it helps with seasonal heaving found in freeze-thaw winter cycles. Tough to find fault with their methods, as it's one ingredient in their recipe. If done properly, the portland/sand base should perform just fine.